Japanese Laid-Open Patent Publication No. 2008-29717 discloses a structure of a shoe sole that urges the rolling action during walk. The shoe sole disclosed in this document has a narrow groove that is continuous from the rear foot portion to the front foot portion.
[First Patent Document] 2008-29717 (Abstract)
However, this conventional technique aims at increasing walking efficiency of elderly people and infants, and would not increase running efficiency as can be seen from description below.
US2007/0193065A1 and WO2006/120749A1 disclose a resin part formed in a U-letter shape extending from the rear foot portion to the middle foot portion.
However, the resin part disclosed in these documents has a generally equal strength on the medial side and on the lateral side of the foot. No longitudinal groove is formed in the front foot portion of the shoes of these documents.
The shoe sole disclosed in WO2008/047538A1 has a depression in the front foot portion and in the rear foot portion. However, the depression in the front foot portion is not parallel to the outer edge of the front foot portion, but is formed along the medial-lateral center line of the front foot portion.
The shoe sole of this document does not have a flexible band-like area in the medial-lateral central portion of the middle foot portion.
U.S. Pat. No. 6,694,642 discloses a cup sole having a through hole in the central portion of the rear foot portion.
The shoe sole disclosed in Japanese Laid-Open Patent Publication No. 2000-333705 has a depression in the central portion of the rear foot portion, and the medial side of the midsole is harder than the lateral side thereof in the middle foot portion. However, no longitudinal groove is provided in the front foot portion.
Japanese Laid-Open Patent Publication No. 2001-346605 discloses forming a groove in the front foot portion of the outsole.